A. Field of the Invention
The invention relates to high voltage, high current fuses, particularly to high voltage, high current fuses having explosive elements therein for interrupting a current path on occurrence of a fault current.
B. Prior Art
The electric power industry frequently requires fuses capable of operating at rated voltages in the kilovolt range and above, and with rated currents on the order of 100 amperes and above. For example, a fuse commonly found at power substations might be required to carry currents of a few hundred amperes at voltages of several kilovolts.
In addition to the need for high relability in such fuses in order to prevent damage to expensive equipment both within the system and connected to it, it is necessary that these fuses respond to fault currents (e.g., currents greatly in excess of the rated operating current of the fuse) in a very short time. For example, in standard power systems operating at sixty cycles per second, the fault current can rise to a peak in as little as 4 milliseconds (that is, one quarter of an AC cycle). Since the destructive power of the fault current is proportional to the product of the square of the fault current and the time during which the current persists, it is essential to limit both the magnitude of the fault current and its duration in order to prevent severe damage to equipment.
In particular, it is desirable to interrupt the fault current in a few milliseconds at most. This requires a very fast response time in order to sense the fault current and provide the necessary interruption before the current reaches a dangerous level. In order to meet these requirements, explosively-assisted fuses have been developed. One example of such a fuse is described in an article entitled "The Development of the Current Limiting Protector (CLP)" by Herbert M. Pflanz, Thomas F. Clark and O. J. Albani, the Conference Proceedings of the IEEE Power Engineering Society, Feb. 1-6, 1981. The fuse there described is formed from a main conductive path for carrying the desired current, a main fuse in parallel with the path, and a number of explosive charges positioned adjacent the main conductive path for severing that path when detonated. An electronic sensor monitors the current in the main conductive path and triggers a detonator when this current exceeds a predetermined level corresponding to the occurrence of a fault.
This system has proven highly useful and effective in limiting fault currents to relatively low values in very short periods of time. However, the need for electronic sensing adds significantly to the cost of the fuse and makes installation and use somewhat more cumbersome due to the need for providing power to the electronics. Further, it requires a variety of different sensor elements, or the provision of means for adjusting the current level at which the sensor triggers, in order to accmodate different rated current levels.